In systems for strictly controlling air flows and static pressures for safety control and quality assurance in research facilities, hospitals, factories, and the like, constant airflow equipment, known as “venturi air valves” are often used in order to achieve room pressure control and local exhaust control.
Venturi air valves, as illustrated in FIG. 12, are constant airflow devices comprising, for example, a venturi tube 101 made from a pipe wherein a narrow section is formed in essentially the center portion thereof; a shaft 102 that is disposed on the inside of the venturi tube 101 so that the venturi tube 101 and the axis line will be coincident, with one end supported on the end portion of the upstream side of the venturi tube 101; a position lever that she supports the other end of the shaft 102 and shifts the shaft 102 in the direction of the axis line; and a cone 104 that is attached to the shaft 102 by a spring 104a. This cone 104 is positioned between a recessed portion of the venturi tube 101 and the upstream side end portion of the venturi tube 101, and a biasing force towards the upstream side of the venturi tube 101 is applied by the spring of 104a. 
In this type of venturi air valve, air flows from the upstream side to the downstream side due to the differential pressure between the upstream side and the downstream side of the venturi tube 101; however, at this time, the amount of flow of the air is maintained at a specific level depending on the position of the position lever 103 if the differential pressure is within a specific range. That is, if the differential pressure becomes larger, for example, the airspeed increases, but because the cone 104 will move towards the downstream side, the cross-sectional area of the gap through which the air flows, between the venturi tube 101 and the cone 104, will become smaller. As a result, the increase in speed of the airflow is offsetted by the amount of reduction in the cross-sectional area of the gap, resulting in the airflow remaining constant. Conversely, when the differential pressure becomes smaller, the speed of flow becomes slower, but the cross-sectional area of the gap through which the air flows between the venturi tube 101 and the cone 104 becomes larger because the cone 104 moves to the upstream side. Consequently, the amount of reduction in the speed of flow is offsetted by the amount of increase in the cross-sectional area of the gap, with the result that the airflow remains constant.
As described above, the cone 104 of the venturi air valve is disposed within the venturi tube 101 that forms the airflow path, and, in order to reduce costs, typically there is no detection of the position of the cone 104. Because of this, in systems that are provided with venturi air valves, the control of the speed of rotation of the fan that draws air from the device wherein the venturi air valve is equipped is controlled through constant rotational speed control (described in, for example, Japanese Unexamined Patent Application Publication No. 2002-39580) without referencing the position of the cone 104, or control is performed so as to maintain a constant static pressure in the main duct.
However, in the conventional system set forth above, the following problems arise when the airflow of the fan has been reduced.
When there is constant rotational speed control, then when, as illustrated in FIG. 13, the airflow is reduced from Q1 to Q2, the operating point of the fan moves from A to A′ (arrow a), not only causing an increase in the head, but also reducing the duct resistance from B to B′ (arrow b). This increased fan head a and the decreased duct resistance (or the head corresponding thereto) β is all absorbed in the venturi air valve or a damper. Consequently, when there is a reduction in airflow from Q1 to Q2, there will be an increase in the resistance of the venturi air valve or the damper equal to α+β.
Additionally, when the static pressure of the main duct is controlled so as to be constant, then, as illustrated in FIG. 13, when there is a reduction in the airflow from Q1 to Q2, the operating point of the fan shifts from A to A″ (arrow c), and so even though the head does not change, the duct resistance will be decreased from B to B′ (arrow b). This change B in the amount of resistance is absorbed in the venturi air valve or the damper. Consequently, when there is a decrease in airflow from Q1 to Q2, then there will be an increase in the resistance of the venturi air valve or the damper equal to β.
In this way, in a system that uses the conventional venturi air valves, when there is a decrease in the airflow of the fan, then the increased head or the decreased duct resistance will be absorbed by the venturi air valve or the duct, which is a waste of the driving force of the fan, and thus, by extension, becomes an impediment to energy conservation.
Additionally, in the conventional system there was either only constant rotational speed control or constant static pressure control, as described above, and thus there was no thought given to structures with branching ducts or to the capacity of multiple venturi air valves. Because of this, it was not possible to perform tight control when applied to ventilating system structures.
Given this, the object of the invention of the present application is to provide a ventilating system and a control method for a ventilating system capable of advancing energy conservation.